1. Field of the Invention
This invention relates generally to improvements in the treatment of municipal sewage and/or industrial wastewater by the activated sludge process. It is particularly concerned with the control of operating conditions to enhance selective production and maintenance in the system of a highly active biomass essentially free from filamentous growth, where the attained sludge has favorable settling characteristics and is capable of substantial removal of phosphate values from the incoming wastewater.
2. Prior Art
The activated sludge process has been used for many years for the removal of biological oxygen demand (BOD) from wastewater. This process consists of maintaining an aeration basin in which wastewater is fed to a suspension of microorganisms to form a mixed liquor. The mixed liquor is aerated to furnish oxygen for the respiration of biomass which sorbs, assimilates and metabolizes the biological oxygen demand of the wastewater.
After a suitable period of aeration, the mixed liquor is introduced to a clarifier in which the biomass settles and the treated wastewater overflows into the receiving stream. A major portion of the settled biomass, which is concentrated at the bottom of the clarifier, is recycled to the aeration basin, and a minor portion purged in order to maintain the constant biosolids inventory within the system. This process has been extensively described in the literature and several of its modifications summarized in a special report on "Wastewater Treatment" by R. H. Marks contained in the June 1967 issue of POWER.
Despite the versatility and effectiveness of this process and its many modifications, there remains a major problem. It is the proliferation at times of high surface area and/or filamentous species, such as sphaerotilus, which do not settle adequately in the clarifier. Thus, one consequence of filamentous biomass is the inability to disengage the biomass from the treated wastewater.
Another problem is effectiveness of removal of phosphate values from the wastewater. There have been isolated instances recorded at various cities within the United States, i.e. Los Angeles, Calif.; Tuscon, Ariz.; San Antonio, Tex.; and Baltimore, Md., wherein phosphate removal has been observed. The reasons for such removal are obscure and the process has been unable to be extrapolated to biological wastewater treatment plants in general. Thus, to date, there are no reliable or satisfactory methods in which the control of phosphate removal by biological action alone has been generally established.
There have been several modifications of the basic activated sludge process aimed at avoiding filamentous and/or high surface area species which result in a phenomenon known as "bulking". One method is to distribute the influent wastewater to different sections of the aeration basin in order to spread out the oxygen demand. Another is to decrease the loading of BOD to the aeration basin. A third is to add poisons to the system in order to selectively kill off the high surface area filaments. Another is to temporarily make the system totally anaerobic and thus kill off the filamentous biomass which is composed largely of obligate aerobes. Another, reported in U.S. Pat. No. 3,864,246, is to maintain a high dissolved oxygen and high steady state BOD level in an initial liquid stage or its hydraulic equivalent in order to promote the selective growth of an active, nonfilamentous biomass which outgrows or precludes the development of filamentous species of higher surface area.
A number of techniques have been proposed in recent years aimed at modification of the conventional activated sludge process to effect or improve removal of nitrogen and/or phosphorous values. Among the various methods disclosed are those including nitrification-denitrification systems, one form of which is that described by Barnard, J. L., Water and Waste Engineering (1974) 33, and subject of South African Pat. No. 7205371. In this process, designated "Bardenpho", four completely mixed activated sludge basins are operated in series followed by a clarifier from which sludge is returned to the first basin. The first and third basins are operated under anoxic conditions, and receive mixed liquor containing nitrates and nitrites (NO.sub.x.sup.-) from the second and fourth basins in the series which are aerated. While periodic good removal of phosphorus and nitrogen in pilot plant operation is reported, when operating at fairly high influent retention time, the production of a dense, easily filterable sludge is not achieved.
None of the methods heretofore proposed are capable of both avoiding bulking biomass and effecting phosphate removal. It is, therefore, among the objectives of the present invention to provide a process which avoids proliferation of filamentous biomass by promoting the growth of a more desirable species which also is capable of removing phosphate values from influent wastewater. In addition, the process of the invention produces an active, dense biomass which concentrates readily in secondary clarification.